Once again I am here with my hat in hand humbly requesting your assistance.
As some of you know, I am in the process of purchasing a Colchester Circa
1960's MK 1 1/2 lathe. It has a two speed 5hp 3 phase motor. Will I be
able to run it successfully on a static phase converter?
Thanks in advance for the help.
I'd say probably, errr welll, make that maybe. Do this: Get in touch with
Phase-A-Matic, large maker of static phase converters. Be prepared to give
them the name plate information from the motor.
I am pretty sure you would be content with the performance of a lathe at
home with a 5 HP 3 phase motor powered with single phase.
You probably wont be content with its performance if you intend to run it
loaded over about 4 HP for extended periods.
I'm impressed with the ability of a lathe like that being able to reverse
Thanks for the help. I don't think I'll be loading it up over 4 HP,
especially for long periods of time. My other concern is whether the two
speed motor will work with a static converter. Some say yes, some say no
and some say maybe.
Reversing could get interesting. That can put a lot of stress on a
phase converter. I've had the situation where a small rotarty would
reverse instead of the lathe, a situation that could really make your
True, but the value of instant reverse is lost. There are times when that
feature is very valuable to the guy on the machine. Tapping a hole with a
tap in the spindle comes to mind. Failure to reverse instantly could
The only 3 phase motor I've not been able to run on a
properly sized static converter is the 2 speed motor on my
Feeler HLVH clone. Whether this is due to something common
to all 2 speed motors, or peculiar to this specific motor,
I can't say.
Bob's suggestion to ask Phase-a-Matic is sound advice.
Another option is a VFD, which is what I did on my lathe.
My lathe also has a two-speed motor, rated at 5 HP on high speed and 3
HP on low speed. I could start it in either mode with a static
converter -- but not with the same static converter, or at least not
with the same capacitance.
There may be a way to trick a relay into the lathe's wiring so when
the high-speed mode is engaged more capacitance is switched into the
Another option would be to manually switch the converter when you
A large enough rotary converter, probably 10 HP idler, would start
it in either mode.
You can build a 10 HP rotary phase converter for next to nothing, in a
couple of days, see how I did it
I am using it very often these days. Works fine, unless it is
overloaded, in which case it is merely mediocre.
With a static phase converter, you will not be able to realize the
full nameplate power of the motor, may have problems reversing it, and
it would run in an unbalanced mode.
Static phase converter is not really a phase converter, it is a
starting circuit that spins up 3 phase motors and lets them run on
single phase once they spin up.
Thus eliminating the "reverse it quite quickly" which was
promised about four articles back-thread. :-)
If you want to do this, you want a hefty rotary phase converter.
I don't think that a "static" phase converter as made by "Phase-O-Matic"
could handle a two-speed three-phase motor very well. It will start
well on one or the other speed, but not both.
According to william_b_noble :
Hmm ... a VFD is not likely to work well with a two-speed
three-phase motor -- unless you do all speed switching with the VFD.
You're not supposed to put the switch between the VFD and the motor, as
the voltage spikes from the switching can zap the output transistors of
Here is a place where I feel that a rotary converter
(build-it-yourself) would be the better choice. Or -- you could limit
yourself to one set of windings on the motor, and do all speed changing
with the VFD. For extreme cases (where you *really* need the higher
torque of the low speed, or the higher speed of the high speed), you can
stop the motor entirely (using the VFD's controls), switch to the other
speed at the lathe's switches, and then start the motor back up with the
I don't think that is the case. All 2 speed motors that I have looked
at have a much lower HP rating on Low speed than high speed. Usually
it is less than 1/2. I would venture that a 2 speed motor runing on
high speed but slowed down with a VFD will actually have more
(or at least as much) HP as the same motor runing on low speed.
So in my opinion you can wire the lathe to only use high motor
speed and then use the VFD to generate the low speed without
any loose in motor torque.
Once you use a VFD you will love it. For example I was facing an
8 inch diameter disc. I set the VFD for 20Hz and geared the lathe
for the speed I wanted for full diameter. As the toolbit
works it way towards the center, I gradually increase the VFD
frequency to increase the spindle RPM. It makes facing a big
object so much faster and better.
I run a 5 horse 2 speed as you are speaking of on a 7.5 horse rotary
built from a phase-a-matic heavy duty static converter rated for 8 hp.
It will NOT start the lathe on the low range but doesn't have any
problems at all with the high range.
Yea, I know...I need bigger. I just wanted to throw out some numbers
that may help. I would suspect that you will not get good service from
a static converter alone. It may be "good enough" for light work in the
speedier gear range but you'll have to seriously oversize it to go into
the low range.
I was able to keep all the original functions of the 2-
speed motor on my lathe when I installed a VFD. The two
speed select contactors are between the VFD and the motor
windings, and work as they always have. The direction
commands to the VFD come from the reversing contactor,
which no longer has any direct connection to the motor.
The only thing I can't do (you *can*, but for the reason
you mentioned, *shouldn't*) is switch from hi to lo on the
fly, but that's not a problem now that speed is
controllable with the VFD's speed pot. Most VFDs have a
zero speed output, so it would be possible to prevent
switching the speed select contactors unless the motor is
stopped, but I didn't think it worth the trouble.
Rather than risk frying the old two speed motor on my TM mill, I got a
1 hp. Marathon inverter duty motor to go with the VFD. Good motors
aren't that expensive, or find a used one. The VFD handles generating
the low speed end well.